Taboo (Reilly Steel #1) by Casey Hill

Unspeakable crimes.
Unimaginable motives.
An unstoppable killer.

Forensic investigator Reilly Steel, Quantico-trained and California-born and bred, imagined Dublin to be a far cry from busting San Francisco, a sleepy backwater where she can lay past ghosts to rest and start anew. She's arrived in Ireland to drag the Irish crime lab into the 21st century, plus keep tabs on her Irish-born father who's increasingly seeking solace in the bottle after a family tragedy.

But a brutal serial killer soon puts paid to that. When a young man and woman are found dead in an apartment, the gunshot wounds on their naked bodies suggest a suicide pact. But Reilly's instincts are screaming that something's seriously amiss, and as more bodies are discovered, the team soon realises that a twisted murderer is at work, one who seeks to upset society's norms in the most sickening way imaginable . . .